Non-volatile memory (e.g., Erasable Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), and flash memory), unlike volatile memory (e.g., read-only memory (RAM)), is capable of preserving information without a power supply. Because non-volatile memory is able to retain data when power is lost, demand for non-volatile memory has increased along with the rise in usage of battery-powered portable electronic devices, such as cellular phones, MP3 players, and digital cameras.
EPROM, EEPROM, and flash memory all utilize floating gate technology in which various methods of gathering charge on the floating gate are employed. For example, Fowler-Nordheim Tunneling (FNT) or Channel Hot Electron (CHE) may be used to surmount the silicon to silicon-dioxide interface. The difference between EPROM, EEPROM, and flash memory lies in how data is erased. An EPROM is usually erased by exposing it to a high dose of UV light, which provides the trapped electrons sufficient energy to escape from the floating gates. Hence, erasing an EPROM requires external equipment and results in deletion of all data stored on it.
In contrast, EEPROM and flash memory can utilize FNT to both write and erase data. Flash memory, however, erases data in chunks (e.g., 512 bytes), rather than 1 cell at a time as in EEPROM. As a result, flash memory has become one of the most popular forms of non-volatile memory since unlike EPROM, it need not be erased in its entirety and does not require external UV equipment for erasure, and unlike EEPROM, it offers much faster erase times.
FIG. 1A shows a circuit diagram of a conventional non-volatile memory cell 100 with a field-effect transistor (FET) 102, a coupling capacitor 104, and a diode 106. FET 102 is coupled to a source terminal 108 and a drain terminal 110. A control terminal 112 is coupled to coupling capacitor 104 and diode 106. In addition, FET 102 and coupling capacitor 104 are coupled to one another.
Illustrated in FIGS. 1B and 1C are cross-sections of FET 102 and coupling capacitor 104 in non-volatile memory cell 100. As shown, FET 102 is an N-type FET comprising a floating gate 114A, a gate dielectric 116A, an N+ source region 118, an N+ drain region 120, N-type lightly-doped drain (NLDD) regions 122, and spacers 124. FET 102 is formed in a P− well 126, which is in a P-substrate 128. Source region 118 and drain region 120 are coupled to source terminal 108 and drain terminal 110, respectively. Isolation regions 130 are formed around FET 102.
Coupling capacitor 104 is an N-type coupling capacitor that comprises an N− well 132 and a control gate 114B separated by a gate dielectric 116B. In FIGS. 1A-1C, floating gate 114A and control gate 114B are electrically coupled using the same conducting trace. An N+ contact region 134 is formed in N− well 132 as a contact for coupling control terminal 112 to coupling capacitor 104. Isolation regions 130 are also formed around coupling capacitor 104. As a result of forming coupling capacitor 104 on P− substrate 128, a PN junction (denoted as diode 106 in FIG. 1A) is formed between P− substrate 128 and N− well 132. To write, read, and erase non-volatile memory cell 100, various voltages can be applied to source terminal 108, drain terminal 110, and control terminal 112 to trap electrons on and remove electrons from floating gate 114A using, for example, FNT.
One drawback of non-volatile memory cell 100 is that when a positive voltage relative to the body of FET 102 is applied to control terminal 112, a depletion region under gate dielectric 116B may form and extend into N− well 132. This decreases the capacitance of coupling capacitor 104 and as a result, higher voltages will need to be applied to write non-volatile memory cell 100.
Another drawback is when source region 118 is positively biased relative to substrate 128 and floating gate 114A, the depletion of NLDD region 122 during erase decreases the electric field appearing across gate dielectric 116A. As a result, a higher voltage will need to be applied at source terminal 108 to achieve FNT from floating gate 114A to source region 118 in order to erase non-volatile memory cell 100.
In addition, the increase in voltage applied may exceed the breakdown voltage (BV) of FET 102. The BV is the voltage at which the junction between the body and the source/drain is subject to an avalanche effect (i.e., when the electric field across the junction is so high that it conducts current via impact ionization), which leads to a breakdown of FET 102. Hence, source region 118 and/or drain region 120 will need to be specially engineered to withstand higher voltages. For example, source region 118 and/or drain region 120 may have to be double-diffused and LDD regions 122 may need to be added (as shown in FIG. 1B). Specially engineered source and/or drain regions, however, not only complicates the fabrication process for non-volatile memory cells, but also slows down programming and erase times of non-volatile memory cells.
Another disadvantage of non-volatile memory cell 100 is that control terminal 112 must be kept more positive than substrate 128 to reverse bias diode 106. If the junction between N− well 132 and P− substrate 128 is forward biased, a substrate current will flow and affect the operation of non-volatile memory cell 100. Therefore, only positive voltages can be applied to control terminal 112 to program non-volatile memory cell 100. As a result, a large voltage must be applied to source terminal 108 or drain terminal 110 to achieve the necessary potential difference with control terminal 112 to erase non-volatile memory cell 100 when control terminal 112 is grounded or kept positive. This, in turn, would also require source region 118 or drain region 120 to be specially engineered with greater breakdown voltage characteristics to withstand large applied voltages.
Accordingly, there is a need for non-volatile memory cells that do not require higher voltages for programming and erasing or specially engineered source and/or drain regions, and have improved programming and erase times. The present invention addresses such a need.